Friction welding metal components

ABSTRACT

A method of forming a structural airframe component for an aircraft and an airframe structural component are provided. The method includes placing at least two components ( 1,2 ) in abutting relationship with each other and joining them together by friction stir butt welding ( 3 ), and the structural airframe component comprises a component manufactured according to the method of the invention.

This application is a continuation of application Ser. No. 09/212,569,filed Dec. 16, 1998, now U.S. Pat. No. 6,328,261 which is a continuationof PCT/GB98/01650 filed Jun. 22, 1998, the entire content of which arehereby incorporated by reference in this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to friction welding of metal, in particularaluminium alloy components, and in particular those used in situationswhere high strength is required such as in structures for aircraft,helicopters, hovercraft, spacecraft, boats and ships.

2. Discussion of Prior Art

Structures and processes of the invention find particular application inaircraft structure, including primary structure, where strength toweight ratio is paramount.

Airframe structural components are inherently complex in their designand subsequent manufacture owing to the large variety of stresses whichwill be applied to the structure in different phases of aircraftoperation, e.g. static, level flight, climb, descent, take-off andlanding or gust conditions. In order to simplify and reduce the numberof airframe components it is a well known principle to integrallymachine from solid billets such components. In this way the parts countsand therefore the weight, cost and complexity of the finished assemblycan be reduced. However limitations upon designs which are achievablecurrently exist owing to restrictions on manufacturing capabilities, forexample in terms of overall billet size combined with the unavailabilityof welded joints for many primary aircraft structures owing to thewell-known fatigue-inducing and crack propagation qualities of weldedjoints.

An example of current design limitations in aircraft wing manufactureoccurs in the available size of upper or lower wing skin panels forconstruction of a wing box. At present, for large passenger-carryingaircraft such as the Airbus A340 family, certain areas of the wing boxrequire a spliced joint between up to four separate machined panelswhere a single panel would be desirable. The overall weight and cost ofwing skins formed by the panels is increased. Also a single panel toreplace the multi-panel assembly would be structurally more efficient.The present limitation on panel size is caused by a limitation on sizeof the aluminium alloy billet from which the panel is rolled.

A further example of the limitations imposed by present technologyoccurs in the manufacture of solid aluminium alloy billets from whichinner wing spars are formed for large commercial aircraft. Any increasein size of such aircraft, as is presently projected for a future largepassenger-carrying aircraft would result in a requirement for a billetlarger than it is currently possible to produce. This restriction raisesthe need for complex bolted joints between components. Such joints willconsiderably increase the weight and complexity of the structure andwill be structurally non-optimum.

Design difficulties can also occur at the intersections between upperand lower wing skins and upper and lower spar flanges respectively in anaircraft wing box. Upper and lower wing skins will be made of differentalloys to enable the different structural requirements to be fulfilled.Where these different alloys are joined to the wing spar fatiguecracking can occur owing to the differing material properties of theskin and spar respectively.

Yet further difficulties can occur in achieving an optimum crosssectional shape at acceptable cost for extruded aircraft wing skinstiffeners, for example stringers. Here the additional material requiredat the ends of the stringers, often called for example “spade ends” or“rib growouts” can dictate the sectional shape for the whole length ofthe stringer and can necessitate machining off unwanted material foralmost the entire length of the stringer, leading to excessively highmachining and material scrap costs.

According to one aspect of the invention there is provided a method offorming a structural airframe component for an aircraft includingplacing at least two components in abutting relationship with each otherand joining them together by friction stir butt welding.

The structural airframe component may comprise an aircraft wing rib andthe at least two components may comprise a central web element and a ribfoot element and the method may include the steps of joining togetherthe central web element and the rib foot element by partial penetrationfriction stir butt welding and subsequently machining away material fromat least one of the central web element and the rib foot element in theregion of the abutment until the weld becomes a full penetration weld.

The method may also include the steps of providing a said rib footelement of L-shape cross section and carrying out the machining away ofmaterial at least from the rib foot element to form a rib foot ofT-shape cross section.

“Butt welding” as used herein is intended to include the process ofwelding together at least two components having edges or surfaces inabutment with each other, whether the components are generally co-planarin the region of abutment or not.

The technique of friction stir butt welding is known from EuropeanPatent No. 615480B assigned to The Welding Institute the entire contentsof which are incorporated herein by reference. The technique involvesplacing the two said components in abutting relationship with eachother, inserting a probe of material harder than the component materialinto a joint region between the two components and causing relativecyclic movement between the probe and components whereby frictional heatis generated to cause portions of the components in the region of thejoint to take up a plasticised condition, removing the probe andallowing the plasticised portions to solidify and join the componentstogether.

The application of this technique to aircraft airframe structure,including primary load bearing structure would not have been foreseenowing to the aforesaid known properties of welds, namely liability tofatigue. Surprisingly however work carried out has revealed that suchfriction stir butt welds do indeed possess the qualities to make suchstructures as aforesaid possible.

In order to exclude the possibility of cracks developing in the regionof the weld joint, a weld fatigue resistant feature may be applied to arun-out of the weld. Such a feature may comprise a cold worked holeformed through the weld joint in the region of the run-out followed byinsertion of a fastener, for example a bolt. Alternatively, or inaddition the joined component in the region of the weld run-out may beshot peened or may have a splice strap fastened in position transverseto the direction of the weld joint. Further in the alternative oradditionally the material of the welded component in the region of theweld run-out may be thickened. By the various above means one of theprimary areas of fatigue of the welded joint may be prevented frombehaving in such an adverse manner.

The friction stir butt welding method may be applied to componentshaving a differing, for example tapering, thickness of material to bewelded together by inserting the said probe into the joint between thetwo components to a depth dependent upon the material thickness at theposition of probe entry. In this way a weld having a penetration throughthe material of the component of sufficient depth to provide aprescribed weld penetration along a length of the weld may be achieved.

The method may include providing a two-piece probe having a centralportion for penetrating the weld region and a peripheral portion movablerelative thereto for travelling over the weld region along surfaces ofthe components being joined, the central portion being movable into andout of the peripheral portion during welding. The central portion andperipheral portion may be relatively movable by a threaded connectiontherebetween or by any other suitable mechanism like a geared connectionor a cam means. The central portion and peripheral portion may includesealing means acting therebetween to prevent ingress of softenedcomponent material.

During movement along the weld of a said component of varying thicknesssuch as a tapered component the rate of feed of the probe along thejoint and rotational speed of the probe may be varied to optimisewelding conditions.

The structural airframe component for an aircraft may comprise a skinstiffener and the method may include the step of placing an extrusion inabutting relationship with an extension or width-increasing region forthe extrusion and joining the extrusion to the extension region byfriction stir butt welding. In this way extension regions such as ribgrowouts and stringer spade ends and other root end profiles of widthlarger than the extruded width may be formed on the stiffener withoutresorting to the formation of an extrusion of the maximum width requiredand then machining long lengths of it away to leave just short lengthsof the extension regions of the extruded width.

According to a second aspect of the invention there is provided astructural airframe component for an aircraft including a friction stirbutt welded joint.

The component in the region of the butt welded joint may be doublecurvature in form. Also, the weld may be of tapering thickness along itslength.

The component may comprise at least two skin panels butt weldedtogether. Wing or fuselage skin or skin/stiffener panels of any requiredsize can thus be produced according to the invention. Said skin panelsmay have a skin stiffener attached thereto along a length of the weld.Such a skin stiffener may comprise a skin stringer in which skinengaging flanges thereof are attached to the skin on either side of theweld.

The component may include a stiffened aircraft wing skin assemblycomprising at least two extruded sections each having integrally formedskin-forming and stiffener-forming portions said sections being weldedtogether. Each weld may include a butt strap fastened thereacross andmay include a run-out feature as described above.

The component may comprise an aircraft skin and stiffener assemblyincluding a joint between two stiffeners at which the skin is frictionstir butt welded together.

The component may comprise an aircraft skin panel having a shaped plugfriction stir butt welded into place therein. In this way localadditions to or thickening of the skin during skin formation may beavoided.

The component may include a friction stir butt welded joint between twosub-components of differing cross section.

The component may comprise a hybrid billet of aluminium alloy comprisingfor example 7000 series alloy friction stir butt welded to 2000 seriesalloy to enable the known properties of these two alloys better to beemployed. In general, friction stir butt welding of hybrid billetsenables tailoring of thicknesses, material strengths and fatigueresistances as required. The billets may be for example forgings orextrusions according to the circumstances.

Where a friction stir butt welded joint replaces a joint usingfasteners, as for example in the manufacture of large aircraft wingspars, there will be a reduction in the number of fasteners usedoverall, with a consequent reduction in cost and weight. Also the weldedjoint itself will be structurally more efficient in that it will bestronger than a fastened joint and, we have discovered, have betterfatigue characteristics. In addition assembly time for a welded jointcan be reduced, owing to elimination of fasteners and joint sealant.Also fuel leakage paths, in a wing, are eliminated.

The component may comprise an aircraft wing rib or spar machined from asaid hybrid billet. The hybrid billet may comprise two or more saidfriction stir butt welded joints positioned on the billet to optimisestrength properties for the billet in the particular circumstancesrequired. For example a said spar may include a weld along a neutralaxis thereof or may include such welds at or in the region of junctionsbetween a central web and upper and lower booms thereof.

The component may comprise an I-section or J-section stiffener of webheight tapering along the length of the stiffener having a friction stirbutt welded joint extending along the length of the tapered web.

The component may comprise an aircraft skin panel having at least onefirst part stiffener formed integral therewith with a further part ofthe stiffener friction stir butt welded to the first part of thestiffener.

The component may comprise an extruded aircraft skin stiffener includingat least one extension region thereof extending the width of thestiffener beyond an extruded width, said at least one extension regionbeing attached to the remainder of the stiffener by a friction stir buttwelded joint. The at least one said extension region may comprise atleast part of a rib growout or a spade end or other root end profileregion of a skin stringer.

The component may comprise an I-section or a J-section skin stiffenerhaving upper and lower booms or flanges separated by a central web andthe at least one extension region may be friction stir butt welded to atleast one of the upper and lower booms on one or both sides of the web.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying drawings of which:

FIG. 1 is a sectional view of a welded stiffened aircraft wing skinpanel assembly according to the invention,

FIG. 2 shows an alternative welded stiffened wing skin panel assemblyaccording to the invention,

FIG. 3 shows a further alternative welded stiffened wing panel assemblyaccording to the invention,

FIG. 4 shows a further alternative welded stiffened wing panel assemblyaccording to the invention,

FIGS. 5A, B and C show alternative methods of welding panel stiffeningmembers according to the invention in position,

FIG. 6 is a sectional view of a welded stiffened panel assemblyaccording to the invention at a joint between stiffeners and panels,

FIG. 7 is a view on the arrow VII of FIG. 6,

FIG. 8 shows in section a junction between a stringer/panel extrudedsection and a further panel extruded section according to the invention,

FIG. 9 is a view in the direction IX on FIG. 8,

FIG. 10 shows a hybrid billet according to the invention,

FIG. 11 shows an aircraft wing spar machined from the billet shown inFIG. 10,

FIG. 12 shows a wing skin-spar assembly welded according to theinvention,

FIG. 13 shows a wing skin-spar assembly as an alternative to that shownin FIG. 12,

FIG. 14 shows a wing skin-spar-rib post assembly according to theinvention,

FIG. 15 is a detail plan view of a stiffened aircraft skin panelaccording to the invention in the region of stringer runouts,

FIG. 16 is a plan view of a friction stir butt weld run-out according tothe invention,

FIG. 17 is a plan view of a alternative friction stir butt weld run-outaccording to the invention,

FIG. 18 is a further alternative view of a friction stir butt weldrun-out according to the invention,

FIG. 19 is a sectional view on the line XIX—XIX of FIG. 18,

FIG. 20 is a side view of a component and probe according to theinvention during the friction stir butt welding process,

FIG. 21 shows, partly in section, a friction stir butt weld probeaccording to the invention,

FIG. 22 shows in section a portion of aircraft skin panel with a buttwelded plug according to the invention inserted therein,

FIG. 23 shows the arrangement of FIG. 22 in subsequent machinedcondition,

FIG. 24 shows a prior art metal billet,

FIG. 25 shows alternative billets according to the invention,

FIG. 26 shows a port wing skin panel according to the invention,

FIG. 27 shows a starboard wing skin panel according to the invention,

FIG. 28 shows a tapered I-section stiffener having a friction stir buttwelded joint according to the invention,

FIGS. 29A, B, C, D, E, show alternative aircraft wing sparconfigurations having friction stir butt welded joints according to theinvention in differing locations,

FIG. 30 shows an aircraft wing spar having a vertical friction stir buttweld according to the invention therein,

FIG. 31 shows an aircraft wing rib having a foot thereof friction stirbutt welded according to the invention to the remainder of the rib,

FIG. 32 shows an alternative arrangement to FIG. 31 according to theinvention,

FIG. 33 shows a friction stir butt welded aircraft wing trailing edgerib according to the invention,

FIG. 34 is an exploded plan view of elements to be used in themanufacture of an aircraft wing rib according to the invention,

FIG. 35 is a view on the arrow XXXV of FIG. 34, with the elementspartially assembled,

FIG. 36 shows the rib of FIG. 34 friction stir butt welded according tothe invention and machined to final shape,

FIG. 37 is a plan view of a typical aircraft wing skin root end stringermanufactured according to the invention,

FIG. 38 is a section along the line XXXIIX—XXXIIX of FIG. 37,

FIG. 39 is a Fatigue Life graph of Maximum Stress plotted against Numberof Cycles to Failure for two plain aluminium alloy specimens, forFriction Stir Welded (“FSW”) aluminium alloy specimens in both “asmanufactured” and machined according to the invention states, and foraluminium alloy specimens with high load transfer joints withinterference fit fasteners, and

FIG. 40 is a graph of Distribution of Residual Stress across the depthof an aluminium alloy plate Friction Stir Welded, plotting ResidualStress at a distance of 10.5 mm from the weld centre against depth.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

FIG. 1 shows skin panels 1, 2 friction stir butt welded together at 3and having a stringer 4 bolted to the skin panels 1, 2 either side ofthe weld 3. A secondary load path is thus provided and the need for abutt strap removed.

In FIGS. 2, 3 and 4 alternative structural assemblies for a wing skin orfuselage skin stiffened assembly are shown. Extruded panel stiffenermembers 5, 6 in FIG. 2 are shown friction stir butt welded together at 3with a butt strap 7 bolted in position to members 5 and 6 either side ofthe weld 3. Again by this means it will be seen that a secondary loadpath is provided.

In FIG. 3 an arrangement similar to that of FIG. 2 includes intermediateskin portions 8 interposed between members 5 and 6 and friction stirbutt welded in position with welds 3. Once again members 5,6 areextruded sections.

In FIG. 4 extruded sections 9, 10 are much wider and each include anumber of stiffening portions 11 but are similarly friction stir buttwelded at 3.

FIG. 5A, B and C show alternative methods of attaching stiffeners topanel members. In FIG. 5A extrusion 12 has a stiffening portion 13,friction stir butt welded to it at 3. In FIG. 5B a friction stir buttweld 3 connects together two panel members 1, 2 and also a T-shapedstiffener member 14. It will be observed that the weld 3 occupies theentire space between members 1, 2 and 14. In FIG. 5C an alternativearrangement to that of FIG. 5B is shown with a T-shaped stiffener 15extending between panel members 1 and 2.

FIG. 6 shows a joint region including a joint 16 between two extrudedaircraft skin panels 17, 18 and two skin stiffeners 19, 20. Straps 21,22 extend between stiffeners 19, 20 and are bolted in position. Inaddition the skin panel members 17, 18 are tapered at 23, 24 to allowthe inclusion of a butt strap 25 positioned over a friction stir buttweld 3 and fastened through the panel members 17, 18. Such a jointarrangement is desirable for example at skin stringer run-outs which canbe conveniently combined with a joint in the panel members 17, 18. Theinherent strength and stability of this joint will be appreciated whichtakes full advantage of the properties of the friction stir butt weld.

FIGS. 8 and 9 similarly show a skin panel joint 26 at run-out of a pairof stiffeners 27, 28. The chain dotted line in FIG. 8 shows wherematerial has been machined away. The components comprise extrudedsections 29, 30 from which material has been machined as describedabove. Section 30 is a combined extruded stringer-panel section andsection 29 is an extruded panel junction section.

In FIGS. 15, 37, 38 two possible designs of aircraft extruded skinstringer are shown, each employing friction stir butt welded extensionregions to widen key parts of the stringer beyond that of the extrusionwidth. In FIG. 15 a stiffened bottom wing skin 110 is shown, havingstringers 111, 112 attached thereto. The stringers each have frictionstir welds 113, 114, 115, 116 which form joints between extrudedsections 117, 118 and extension regions 119, 120, 121, and 122respectively. The extruded width W1 of the stringers is seen to be muchnarrower than the final width W2 and prior art manufacturing methodswould have required the stringers to have been extruded to width W2,thus requiring the machining off of scrap regions 123, 124, 125, and 126for almost the whole length of the stringers, save for the end regionsas shown and for any “rib growouts”.

FIGS. 37 and 38 show two views of an aircraft wing root end stringer andskin assembly in which a root end profile of the stringer 127 has beenextended in a similar manner to stringers 111 and 112 of FIG. 15 by theattachment of extension regions 128, 129, 130 and 131 to an extrudedsection 132 by friction stir welds 133, 134, 135 and 136. By this meansthe J-section of the extruded section 132 has been converted to anI-section of the root end profile to provide the added stiffnessrequired in the runout part of the stringer. It will be appreciated thatpotentially even greater savings of material and machining time can beachieved in this example as both upper and lower booms of the stringerwould require machining along almost their entire length according tothe prior art.

In FIG. 10 aluminium alloy hybrid billet 31 is shown formed of 7000series and 2000 series alloy. When machined this billet takes up theform shown in FIG. 11 with the advantages of the different alloyproperties being readily apparent to the skilled reader when such a spar32 extends between upper and lower wing skin panels (not shown)positioned along edges 33, 34 respectively and themselves havingdifferent material properties with the upper skin typically being 7000series aluminium alloy and the lower skin typically being 2000 seriesaluminium alloy.

In FIG. 12 part of a wing spar 35 is shown friction stir butt welded at36 to an angled skin portion 37 which is in turn friction stir buttwelded at 38 to a skin panel member 39. A separate rib post 40 isfastened in position to spar 35 and angled portion 37. This exampleteaches how a construction according to the invention works to thedesigners advantage in designing a complex structural joint assembly. Inthis example the portion of spar 35 shown comprises 7000 seriesaluminium alloy, the angled portion 37 comprises 2000 series aluminiumalloy and the skin portion 39 comprises 2000 series aluminium alloy.

In FIG. 13 an extruded skin stiffener portion 41 is friction stir buttwelded at 42 to a 2000 series aluminium alloy lower portion 43 of a wingspar. This in turn is friction stir butt welded at 44 to a 7000 seriesaluminium alloy upper portion 45 of the spar. This arrangement givesvarious manufacturing advantages without losing strength of theassembly.

In FIG. 14 an extruded part stiffened skin member 46 is friction stirbutt welded at 47 to a skin panel 48. Skin panel 48 is of 2000 seriesalloy and the member 46 comprises 7000 series alloy. A rib post 49 isfastened conventionally to stiffening portion 50 of member 46. Sealantis applied at 51.

In FIG. 16 a run-out area of a friction stir butt weld 52 is shown. Theweld 52 extends between two panel members 53, 54. A cold worked hole 55has been drilled through the weld for insertion of a bolt therethrough.An edge 56 of panels 53, 54 has been shot peened. The overall result ofthis arrangement is a friction stir butt weld run-out of great safety,having residual stress removed with consequentially improved fatiguelife.

FIG. 17 shows a similar arrangement having a splice strap 57 extendingacross the weld 52. Again edges 56 have been shot peened.

In FIGS. 18 and 19 an arrangement similar to that shown in FIG. 17includes panels 53, 54 being thickened at portion 58 further to increasethe strength of the weld at the run-out. In FIG. 19 height H of thepanels 53, 54 is shown increased at the edge 56. In addition the splicestrap 57 is shown tapered in form.

FIG. 20 shows an adjustable friction stir butt welding tool 59 having aninner portion 60 and an outer portion 61. The tool is shown traversingbetween two panels 62, 63 which are tapered in section. It will beobserved that the upright rotational axis of the tool 59 is tilted fromthe vertical to accommodate the taper in the panels. The inner portion60 of the tool is retractable with respect to the outer portion 61. Inthis way weld depth can be varied to accommodate taper of the workpiecepanels. With this arrangement dynamic tool control of temperature, feed,rotational speed may be monitored and varied for optimum weld jointproperties.

FIG. 21 show the tool 59 of FIG. 20 partly in section and shows how theinner portion 60 is threaded with respect to the outer portion 61. Aseal 64 between the inner and outer portion is shown to prevent ingressof workpiece material.

FIGS. 22 and 23 show a skin panel 65, an aperture 66 cut therein and acircular plug 67 friction stir butt welded into position at 68. Thefinished component as machined is shown in FIG. 23. Here a small amountof excess material has been machined away from a periphery of the plug67 and a bore 150 has been formed in the plug. By these means a panelmay be manufactured in the first instance of much lesser thickness,avoiding the need to machine away large amounts of material from allsave the small area 151 surrounding the bore. It will be appreciatedthat the plug 67, although circular in cross section in this example,may be of any required shape.

In FIG. 24 according to the prior art a billet 69 of aluminium alloymaterial is shown from which port and starboard wing skin panel sections70, 71 of FIGS. 26, 27 may be machined. It will be appreciated that thehatched part of the billet of FIG. 24 will be wasted in all cases. Usinga method according to the invention however, as shown in FIG. 25 andalso in FIGS. 26 and 27, each panel 70, 71 may be made up from billets72, 73, 74, 75 respectively which are then friction stir butt weldedtogether at 76, 77. In this way port and starboard wing skin panels maybe efficiently made and the wasted hatched part of the billet of theprior art eliminated.

In FIG. 28 is shown a tapered stiffener section 78 having a frictionstir butt welded joint 79 centrally thereof. Tapered sections may bemore efficiently produced in this way.

FIGS. 29A, B, C, D and E show alternative aircraft wing sparconfigurations with friction stir butt welds 80, 81 at various locationsthereof in order to optimise strength, manufacturing requirements asnecessary. It will be appreciated that each portion of the spar may beof a different material to allow for say better tension properties in alower portion and better compression qualities in an upper portion.

In FIG. 30 is shown a wing spar 82 having a friction stir butt weldedjoint 83 vertically disposed thereon for additional strength. Thisallows the insertion of structurally more optimum sections into highlyloaded areas.

FIG. 31 shows a wing rib having a friction stir butt weld 84 forming ajoint between an upper portion 85 and a lower portion 86 thereof. Thisconstruction method obviates the need for machining from a solid billetand affords enormous material savings in comparison thereto.

FIG. 32 shows an alternative wing rib design with a rib foot 87manufactured from two parts 88, 89 friction stir butt welded together at90. An additional friction stir butt weld 91 joins portion 89 with a webportion 92.

In FIG. 33 an aircraft wing trailing edge rib is shown having frictionstir butt welds 93, 94, 95 to considerably reduce manufacturing cost andmaterial wastage by avoiding the machining away of large amounts ofscrap material from a solid billet.

In FIGS. 34, 35 and 36 an aircraft wing rib and method of manufacturingit are shown. In FIGS. 34 and 35 a central billet 96 from which will bemachined a stiffening web 97 is shown ready for attachment, by frictionstir butt welding, of a series of outer billets 98, 99. In FIG. 35 theelements 96, 98, are shown placed in abutment to each other with arotating probe 100 of a friction stir welding tool 101 in the process ofpartial penetration welding them together to form a weld 105. Theelements 96 and 99 have already been welded together with a partialdepth friction stir weld 102. It will be noted from FIGS. 35 and 36that, although outer billets 98, 99 are of L-shaped cross section, ribfeet 103, 104 are of T-section. Also, although the welds 102 and 105,are formed as partial penetration welds, once the billets 96, 98, 99have been machined to their Final shape shown in FIG. 36, the welds 102,105 have become full penetration welds. At least two benefits flow fromthis method of manufacture. Firstly, the tooling (not shown) requiredfor jigging the billets 98, 99 is greatly simplified as compared to thatwhich would be required for jigging parts of the T-shape of the rib feet103, 104; the billets 98, 99 may be simply held down against the surface106 of a bed 107 by clamps acting upon top surfaces 108, 109 of thebillets 98, 99 respectively, similarly for billet 96. Secondly, thewelds 102, 105 may be stronger than otherwise owing to the greater massof metal of billets 98, 99 acting to conduct heat away faster from thewelds 102, 105 and reducing the heat affected zone surrounding the welds102, 105. It will be appreciated that rib feet 103, 104 would have hadto be machined from billets of the shape of billets 98, 99 in any event.In addition dimensional tolerancing of the rib feet 103, 104 may be madeeasier to achieve owing to all machining being carried out on the rib inits final assembled and welded state.

The L-shaped billets 98, 99 could alternatively be extrusions. Inaddition, adjoining billets 98 and 99 may be welded to the billet 96 inseparate sections or several or all billets 98 or 99 may be joined toeach other prior to welding to billet 96. As a further alternative thebillets 98, 99 could each comprise a single shaped billet. This could beforged, machined from plate or extruded for example and formed to thedesired curvature and friction stir butt welded to the billet 96. A partreduction from 8 pieces to 2 would result.

It will be appreciated that where the use of extruded sections isallowed by constructions of the invention enormous savings in materialcosts are possible with corresponding savings in assembly costs wherecomponent count is reduced. The use of friction stir butt welding inaircraft airframe structures enables the use of extruded sections, forexample wing skin-stiffener sections where they have not been possiblebefore, see in particular FIGS. 2, 3, 4.

Referring to the graph of FIG. 39, Max. Stress in MPa is plotted againsta logarithmic scale of Number of Cycles to Failure for five cases. Thethree uppermost curves, marked “Naval Research Lab Data” and “MIL-HDBKData” plot the aforesaid variables for aluminium alloy 2024-T3 plainspecimens. These curves show the longest fatigue life for given stresslevels up to approximately 7–800,000 cycles, however the curve for themachined FSW specimen crosses two of these plain specimen curves beforethe maximum measured number of cycles to failure of 10,000,000 to giveresults superior to them both. The teaching that can reliably be takenfrom this graph is that clearly the FSW specimen performs extremely wellin comparison to the plain specimens and that this performance improvesas maximum stress reduces. The curves for the “as manufactured” FSWwelded specimin and for the specimen with high load transfer joints withinterference fit fasteners perform markedly below those above.

It was observed with very considerable surprise therefore that thespecimen with the machined FSW joint performed between 75% and 100+% aswell as the tested plain specimens in this standard fatigue test,particularly as the unmachined FSW specimen had performed (as expected)less well than the plain specimens. It should be noted however that theperformance of the unmachined FSW specimen closely matched that of thespecimen with high load transfer joints with interference fit fastenersand therefore surprisingly proved itself suitable for use in airframestructural components.

Referring to FIG. 40, it has now become clear why the machined FSWspecimen performed with such excellence in the fatigue test recorded inFIG. 39. In FIG. 40, which looks at residual Stress at different depthsfrom the surface, the peak value of tensile/positive stress occurs inthe longitudinal direction and registers approximately 300 MPa. It willbe seen that this figure sharply reduces to approximately 200 MPa by thedepth of approximately 0.10 mm however, and further again at a depth ofapproximately 0.25 mm until it reaches its lowest level of aproximately130 MPa at a depth of approximately 0.50 mm. From this depth there isapparently little to be gained from further machining of the surface andthe residual stress remains in the region of 140–150 MPa thereafter.

From the foregoing it will thus be clearly appreciated that “asmanufactured” or unmachined FSW is, surprisingly, perfectly suitable foruse in structural airframe components for aircraft, exhibitingcomparable fatigue life to HLT joints with interference fit fasteners,but offering the designer the major potential advantages of lowerweight, reduced parts count and assembly time and large savings inmachining time and material scrap. However machined FSW offers evengreater fatigue life, exhibiting for 2024 aluminium alloy approximately75% of the fatigue life of plain 2024 material. From work on otheraerospace aluminium alloys it is believed that the fatigue lives ofthese alloys too will be similarly enhanced when FSW is used. Thisfigure is much higher than is obtainable for HLT joints withinterference fit fasteners and therefore offers the designer enhanceddesign scope, both as discussed above and in tailoring billets tospecific requirements by the use of FSW joints between different alloytypes, when this type of welded structure is used.

1. A structural airframe component for an aircraft including at leastone friction stir butt welded joint, in which the thicknesses of thecomponent materials being joined have different cross sections.
 2. Astructural airframe component as in claim 1 in which the componentcomprises at least two skin panels friction stir butt welded together.3. An airframe for an aircraft including at least one structuralairframe component according to claim
 2. 4. A structural airframecomponent as in claim 1 in which the at least one friction stir buttwelded joint joins at least two extruded integrally-stiffened wing panelsections.
 5. An airframe for an aircraft including at least onestructural airframe component according to claim
 4. 6. A structuralairframe component as in claim 1 in which the said weld joins a wingskin panel and one of a spar and rib.
 7. A structural airframe componentas in claim 6 in which a part of the said one of a spar and rib formspart of an aerodynamic profile of the wing.
 8. An airframe for anaircraft including at least one structural airframe component accordingto claim
 7. 9. An airframe for an aircraft including at least onestructural airframe component according to claim
 6. 10. An airframe foran aircraft including at least one structural airframe componentaccording to claim
 1. 11. A structural airframe component for anaircraft including at least one friction stir butt welded joint in whichthe at least one friction stir butt welded joint joins at least twoextruded integrally-stiffened wing panel sections.
 12. An airframe foran aircraft including at least one structural airframe componentaccording to claim
 11. 13. A structural airframe component for anaircraft including at least one friction stir butt welded joint in whichthe said weld joins a wing skin panel and one of a spar and rib.
 14. Astructural airframe component as in claim 13 in which a part of the saidone of a spar and rib forms part of an aerodynamic profile of the wing.15. An airframe for an aircraft including at least one structuralairframe component according to claim
 14. 16. An airframe for anaircraft including at least one structural airframe component accordingto claim
 13. 17. A structural airframe component for an aircraftincluding at least one friction stir butt welded joint wherein, in theregion of a said butt welded joint, the component is double curvature inform in which the thickness of material being joined varies across theweld joint.
 18. An airframe for an aircraft including at least onestructural airframe component according to claim
 17. 19. A structuralairframe component for an aircraft including at least one friction stirbutt welded joint wherein, in the region of a said butt welded joint,the component is double curvature in form in which the at least onefriction stir butt welded joint joins at least two extrudedintegrally-stiffened wing panel sections.
 20. An airframe for anaircraft including at least one structural airframe component accordingto claim
 19. 21. A structural airframe component for an aircraftincluding at least one friction stir butt welded joint in which thecomponent comprises at least two skin panels friction stir butt weldedtogether in which the at least one friction stir butt welded joint joinsat least two extruded integrally-stiffened wing panel sections.
 22. Anairframe for an aircraft including at least one structural airframecomponent according to claim
 21. 23. A structural airframe component foran aircraft including at least one friction stir butt welded jointwherein, in the region of a said butt welded joint, the component isdouble curvature in form in which the said weld joins a wing skin paneland one of a spar and rib.
 24. A structural airframe component as inclaim 23 in which a part of the said one of a spar and rib forms part ofan aerodynamic profile of the wing.
 25. An airframe for an aircraftincluding at least one structural airframe component according to claim24.
 26. An airframe for an aircraft including at least one structuralairframe component according to claim 23.